Remotely operated seismometer

ABSTRACT

A seismometer suitable for remote operation to measure quake components, comprising a boom mounted for pivoting about a nominally vertical or other axis, the boom carrying a sensing coil and a small iron slug. A pair of calibration coils mounted on opposite sides of the slug can be individually energized to deflect the boom in opposite directions from its static position. The sensing coil outputs resulting from separate energizations of the calibration coils, indicate the precise static position of the boom and therefore, in the case of a boom intended to pivot about a vertical axis, the angle of its nominally vertical axis of pivoting from a true vertical. The sensing coil is flat and is surrounded by a frame of non-conductive material with conductive layers on opposite sides that are electrically connected to the ends of the coil. The frame is supported by a pair of mass members which are insulated from each other but connected to the respective conductive layers on the frame. The mass members are mounted on separate Cardin hinges, and currents generated by the sensing coil are transmitted through the Cardin hinges to output terminals.

Lehner et al.

tates REMUTELY OPERATED SIEHSMOME'KER Assignee:

Filed:

Inventors: Francis E.

Lehner, Monrovia;

Wayne F. Miller, Arcadia, both of Calif.

California lnstitute of Technology,

Pasadena, Calif.

Aug.13,1970

Appl. No.'. 63,512

US. Cl. ..340/l7, 73/1 D, 73/517 R,

lint. Cl. ..G0lv 1/16 Field oi Search....340/l7; 73/1 D, 5 I? B, 518 R,

Refierences Cited UNITED STATES PATENTS l2/l968 6/l970 l/l967 ll/l9665/l957 Primary Examiner Benjamin A. Borchelt Assistant Examiner-R.Kinberg Attorney-Lindenberg, Freilich & Wasserman [57] ABSTRACT Aseismometer suitable for remote operation to measure quake components,comprising a boom mounted for pivoting about a nominally vertical orother axis, the boom carrying a sensing coil and a small iron slug. Apair of calibration coils mounted on opposite sides of the slug can beindividually energized to deflect the boom in opposite directions fromits static position. The sensing coil outputs resulting from separateenergizations of the calibration coils, indicate the precise staticposition of the boom and therefore, in the case of a boom intended topivot about a vertical axis, the angle of its nominally vertical axis ofpivoting from a true vertical. The sensing coil is flat and issurrounded by a frame of non-conductive material with conductive layerson opposite sides that are electrically connected to the ends of thecoil. The frame is supported by a pair of mass members which areinsulated from each other but connected to the respective conductivelayers on the frame. The mass members are mounted on separate Cardinhinges, and currents generated by the sensing coil are transmittedthrough the Cardin hinges to output terminals.

10 Claims, 5 Drawing Figures PATENTEDJM 23 I913 SHEET 1 OF 2 Mania/sLam/5f? WflVA/ F M/LLEY INVENTORS REMOTELY OPERATED SEISMOMETER ORIGINOF INVENTION The invention described herein was made in the performanceof work under NASA contract and is subject to the provisions of Section305 of the National Aeronautics and Space Act of I958, Public Law 85-568(72 Stat. 435; 42 USC 2457).

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to seismometers.

2. Description of the Prior Art Most seismometer installations requirethree seismometers for measuring three orthogonal components of quakes,two of the seismometers generally measuring horizontal components. Manyseismometers employ pivoting booms, and when such seismometers are usedto measure horizontal quake components, they are set up so that the boomcan pivot about a vertical axis. Where an operator is present, he canmanipulate leveling feet to orient the axis of pivoting in a preciselyvertical direction. However, in those applications where an operator isnot available, as in the case of unmanned craft which are intended torecord quakes on the moon or planets, there is no operator available toperform such leveling. Instead of resorting to complex automaticleveling devices, the amplitude and direction of quakes can bedetermined by merely correcting for deviation from accurate leveling.Apparatus which could indicate the degree of such deviation, but whichadded a minimum of complexity to the equipment, would be desirable forsuch seismometers. Of course, a construction of such seismometers whichresulted in the greatest compactness and ruggedness is also required onunmanned extra-terrestrial missions.

OBJECTS AND SUMMARY OF THE INVENTION An object of the present inventionis to provide a seismometer with a pivotally mounted'boom, which, inaddition to sensing ground oscillations, indicates the deviation of theaxis of pivoting of its boom from the vertical.

Another object is to provide a highly compact and rugged seismometer.

In accordance with one embodiment of the present invention, a compactand rugged seismometer is provided which can indicate the accuracy ofits leveling to an operator at a remote station. The seismometerincludes a boom that is pivotally mounted on a main frame to pivot abovea nominally vertical axis, the boom carrying a sensing coil that movesbetween a pair of magnets so that motion of the boom causes a current tobe generated in the coil. The boom also carries a small iron slug, andthe main frame carries a pair of calibration coils that are located onopposite sides of the slug. Currents to the different calibration coilstend to pivot the boom in opposite directions, the amounts of pivotingbeing equal if the pivotal axis of the boom is precisely vertical.However, if the pivotal axis is angled from the vertical, thenenergization of the different calibration coils results in differingamounts of pivoting of the boom. The difference in the amount ofpivoting indicates the precise misalignment of the axis from thevertical.

The boom of the seismometer has a flat coil surrounded by a coil frameof insulating material which is plated with a conductive layer onopposite faces thereof. The ends of the coil are connected to theopposite conductive layers of the coil frame, so that connections to thecoil can be made through the more rugged conductive layers of the framethan to the delicate wire of the coil. The coil frame is supported by apair of mass members which are disposed against the conductive layers onits opposite faces to make contact therewith, the mass members beingconstructed of conductive material and being insulated from each other.Each of the mass members is mounted on a separate hinge, and current isconducted through the hinge to terminals on a main frame that pivotallysupports the boom. The conduction of current through the hingeseliminates the need for delicate wires from the boom to the main frame,which could easily be broken.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will be best understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of aseismometer mechanism constructed in accordance with the presentinvention;

FIG. 2 is a perspective view of the boom of the seismometer mechanism ofFIG. 1;

FIG. 3 is a plan view of the seismometer mechanism of FIG. 1;

FIG. 4 is a view taken on the line 4-4 of FIG. 3; and

FIG. 5 is a graph showing the calibration characteristics of theseismometer mechanism of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates aseismometer mechanism for measuring horizontal components of quakes inthe direction of arrows 10, the seismometer including a boom 12 which ispivotally mounted about an axis 14 on a main frame 16. The boom 12includes a sensing coil 18 held by a coil frame 20, the coil frame beingheld by a pair of mass members 22, 24 that are pivotally mounted on themain frame by a pair of Cardin type hinges 26, 28. The main frame 16carries a pair of permanent magnets 30, 32 which are disposed onopposite sides of the coil 18, so that when the sensing coil moves asthe boom pivots, currents are generated in the coil. These currents arecarried from the coil 18 to a pair of sensing coil output terminals 34on the main frame. The seismometer can be used in remote operations suchas in craft that are landed on the moon or other planets to measurequakes thereon, and the terminals 34 may be connected to a telemetrysystem for transmitting information about the currents to a remotestation such as one on the Earth.

The seismometer mechanism shown in FIGS. 1, 3 and 4 is generally one ofthree seismometers in an assembly for measuring quake components inthree orthogonal directions, two of the seismometers being used tomeasure horizontal components and and the other being used to measurevertical components. In order to measure the horizontal components ofquakes in the direction of arrows 10, the pivotal axis 14 of the boomshould be oriented in a precise vertical direction be possible toprovide an automatic leveling system for pivoting the mechanism so thatthe axis 14 is precisely aligned with the vertical, but this wouldrequire a system of substantial complexity and added weight. In-

stead of reorienting the seismometer, it is constructed so thatinformation can be transmitted to indicate the precise angle ofmisalignment of the axis 14 with the vertical. This allows operators atthe remote station to compensate for the misalignment so that any quake3 measurements can be compensated to yield accurate information aboutany quake occurrences.

In accordance with the invention, information about the orientation ofthe seismometer is obtained by the use of a pair of calibration coils36, 38 mounted on the main frame and an iron slug 40 mounted on the boombetween the calibration coils. The calibration coils 36,"

38 are substantially identical, and have cores in the form of ironscrews 42, 44 that can be turned to move towards or away from the ironslug 40 for calibrating the device. If the seismometer is oriented withaxis 14 precisely vertical, then the boom 12 will assume a predeterminedcentral position as shown in FIG. 1. With the boom in this centralposition, the mechanism may be initially adjusted so that the cores 42,44 are at the same distance from the slug 40. A current pulse througheither calibration coil 36 or 38 will then pivot the boom 21 inprecisely the same amount, although in opposite directions. If the axis14 is angled with a component in the direction of either arrow away fromthe vertical, then the boom will assume a rest position wherein the slug40 will be closer to one calibration coil than the other. If a pulse isthen sent through the calibration coils in turn, the calibration coilwhich is closer to the slug 40 will produce a larger swing of the boomthan will be produced by a current pulse of equal magnitude applied tothe other calibration coil. The magnitude of this different in swingproduced by a pulse of given magnitude indicates the degree ofmisalignment of the axis of pivoting 14 of the boom from the localvertical. The amount of pivoting of the boom is determined by thecurrent generated by the sensing coil 18 which generates a current as itmoves between the permanent magnets 30, 32.

The graphs of FIG. 5 illustrate the change in boom swing resulting fromtilting of the axis of pivoting 14 away from the vertical. The swing ofthe boom is given by the peak output from the sensing coil 18 whichgenerates a current pulse as the boom swings. The graph 50 shows therelationship between the sensing coil output and the tilt of the pivotalaxis when a current pulse of predetermined magnitude is passed throughthe calibration coil 36. Graph 52 illustrates the relationship betweensensing coil output and tilt when the calibration pulse is sent throughthe other calibration coil 38. When the seismometer is level, so thataxis 14 is vertical, the sensing coil output is at the level 1.0 for apulse in either calibration coil 36 M38. When the pivotal axis 14 tiltsin a positive direction degrees tilt on the graph) which moves the slug40 further away from calibration coil 36, then a pulse in that .coilproduces a smaller swing of the boom, while a pulse in the other coilproduces a larger boom swing and vice versa. The calibration curve ofFIG. 5 may be produced by measurements in Earth gravity, and may beadjusted to give accurate indications of tilt angle for any other .knowngravity strength, such as the strength on the moon or on another planet.7

The seismometer is constructed for extreme compactness and ruggedness.As shown in FIG. 2, the

sensing coil 18 is wound about an elongated core to build up arelatively flat coil. After the coil is wound, it is mounted on the coilframe 20 by cementing it in place. The frame 20 has a body 62 ofelectrically insulating material, and has a layer or film 64, 66 oneither side of the electrically conductive material, the frame beingconstructed in the same manner as typical etched circuit boards. Theends of the coil 18 are electrically connected to the conductive layers64, 66. The mass members 22, 24 which provide most of the mass of theboom, are constructed of electrically conductive material of at leastmoderately high density, such as brass. They are joined to oppositesides of the coil frame20, and each is in electrical contact with one ofthe conductive layers 64 or 66 on the coil frame. Each mass member hasan inner end 68, 70 which is connected by one of the Cardin hinges 26,28 to the main frame of the seismometer.

Each Cardin hinge includes a pair of electrically conductive resilientmembers ofia material such as spring steel, each resilient member havingone end fixed to the boom and the other end fixed'to the main frame. Thetwo elastic members of each hinge extend perpendicular to each other,and therefore are seen to cross one another as viewed along the axis ofthe hinge, as in FIG. 3, or in other words, they are crossed. As shownin FIG. 1, a pair of mounts 72, 74 are provided on the main frame forholding those ends of the Cardin hinge'members which are fixed relativeto the main frame. These mounts 72, 74 are connected to the two outputterminals 34 on the main frame from which sensing coil currents aredelivered to data processing equipment or the like. Thus, currentgenerated by the sensing coil 18 passes along the two conductive layers64, 66 on the coil frame, through the two mass members 22, 24 that holdthe coil frame, through the Cardin hinges 26, 28 which mount the massmembers on the main frame, and through the mounts 72, 74 to the outputterminals 34.

It would be possible to run a pair of wires directly from the sensingcoil 18 to the output terminals 34, to eliminate the need for electricalconnections through the hinges. However, such wires would have to bevery thin to prevent interference with swinging of the boom, and suchthin wires could be easily broken. By using two Cardin hinges which areelectrically insulated from each other and transmitting current throughthem, the need for delicate wires to conduct current from the boom iseliminated.

.The boom could be constructed with'the ends of the sensing coil 18connected directly to the mass members 22 and 24 instead of throughconductive layers on the coil frame 20. However, the coil 18 isgenerally wound with extremely fine wire such as wire of onethousandthinch diameter, and it could be damaged before connection to the massmembers. Instead, the coil is mounted on the coil frame 20 immediatelyafter it is wound so that the entire coil and frame may be thereafterhandled as a unit. The conductive layers 64, 66 on the coil frame aremuch more rugged than a free standing wire, and the coil frame can besecurely connected to the mass members 22, 24.

Each of the mass members 22, 24 has a web portion 76, with a width andlength each at least twice as great as its thickness, that connects mostof the mass of the mass member to the inner end 68 or 70 thereof. Thewebs 76 are oriented so that imaginary lines normal to their faces aresubstantially parallel to the axis of pivoting 14 of the boom. Thereason for providing the webs 76 is to allow the mass members 22, 24 todeflect sidewardly in a direction perpendicular to the usual directionin which the boom pivots. lfrthe seismometer is jolted, and the massmembers 22, 24 tend to move sidewardly until the coil frame hits amagnet, the webs 76 deflect and allow such movement to occur withoutexcessively straining the Cardin hinges, thereby protecting the hingesagainst damage. Stops 78, 80 are also provided to prevent excessive boompivoting about its axis of pivoting 14.

The iron slug 40 which interacts with the calibration coils 36, 38 ismounted on a strip 82 of insulating material, which extends between theinner ends 68, 70 of the mass members. The strip 82 is located so thatthe slug 40 and the calibration coils that interact with it are on aside of the axis of pivoting l4 opposite the side where the sensing coil18 and the permanent magnets 30, 32 are located. This places thecalibration coils 36, 38 away from the sensing coil 18 to minimize theinfluence of the magnetic field produced by the calibration coils on thesensing coil. It may be noted that the calibration coils are used notonly to sense leveling, but to measure the amount of pivoting of theboom for a given impulse to it, to verify the operating characteristicsof the instrument. The seismometer is generally transported and landedwith the boom clamped in a predetermined position to prevent itsmovement. FIG. 3 shows a clamp plunger 84 which bears against the boomto prevent pivoting. After the instrument has been landed, the clamp isreleased and calibrations may be made to determine whether the boom isfree to pivot and to determine the leveling of the seismometer. Theseismometer has three calibration coil terminals 86 through whichcurrents are delivered to the calibration coils on command.

Thus, the invention provides a compact and rugged seismometer whichenables a remote determination of seismometer leveling. A determinationof leveling is achieved by the use of a pair of calibration means forurging the boom to pivot in opposite directions, the amount of boommovement resulting from such impulses indicating the degree of leveling.The rugged-ness of the seismometer is increased by eliminating thinfree-standing wires for connecting a coil on the boom to the main frame,and instead connecting the coil through a pair of hinge members thatpivotally support the boom. It may be noted that a similar seismometerconstruction can be employed for a seismometer that measures verticalquake components and whose boom 6 is pivotally mounted about a nominallyhorizontal axis. A pair of calibration coils then indicates the angle ofthe axis from a true horizontal.

Although particular embodiments of the invention have been described andillustrated herein, it is recognized that modifications and variationsmay readily occur to those skilled in the art and, consequently, it isintended that the claims be interpreted to cover such modifications andequivalents.

What is claimed is:

l. A seismometer comprising:

a main frame;

a boom;

means for mounting said boom on said frame to allow pivoting of saidboom with respect to said frame about a nominally vertical axis whileresiliently urging said boom toward a predetermined pivotal position;

means for sensing the amount of pivoting of said boom; and

first and second calibration means for urging said boom to pivotrespectively in first and second opposite directions, irrespective ofthe position of the boom whereby to enable a measurement of theorientation of the axis of pivoting of said boom with respect to thevertical.

2. A seismometer comprising:

a main frame;

magnet means mounted on said frame;

a flat substantially spiral coil;

a coil frame of electrically insulative material extend ing about mostof the periphery of said coil and supporting it;

a pair of mass members mounted at opposite faces of said coil frame,said mass members constructed of electrically conductive material,electrically insulated from each other, and electrically coupled todifferent portions of said coil;

a pair of hinges for pivotally coupling said mass members to said mainframe so that said coil is positioned adjacent to said magnet means,said hinges constructed of electrically conductive material,electrically insulated from each other, and each electrically coupled tothe mass member which it pivotally couples to said main frame; and

a pair of output terminal means coupled to said hinges.

3. The seismometer described in claim 2 wherein:

said coil frame has electrically conductive layers on its opposite facesthat are directly connected to different portions of said coil; and

each of said mass members is connected to a different one of saidconductive layers, whereby to facilitate construction of theseismometer.

4. A seismometer comprising:

a frame;

magnet means mounted on said frame;

a boom including a sensing coil and a portion of magnetically attractivematerial spaced from said coil;

means for pivotally mounting said boom on said frame so that saidsensing coil is urged toward a predetermined pivotal position where itlies adjacent to said magnet means while said portion of magneticallyattractive material is spaced from said magnet means;

first and second calibration coils mounted on opposite sides of saidportion of magnetically attractive material for respectively urging saidboom to pivot in opposite directions; and

sensing terminal means electrically coupled to said sensing coil. Theseismometer described in claim 4 wherein:

said sensing coil and said portion of magnetically attractive materialare on diametrically opposite sides of the axis of pivoting of saidboom.

6. A seismometer comprising: a frame; magnet means mounted on saidframe; and

7. A seismometer comprising:

amain frame;

aboom;

means for mounting said boom on said frame to allow pivoting of saidboom with respect to said frame about a nominally vertical axis whileresiliently urging said boom toward a predetermined pivotal position;said boom having first and second portions on opposite sides of its axisof pivoting, and said second portion including magnetically attractivematerial;

means for sensing the amount of pivoting of said boom, including asensing coil mounted on said first portion of said boom, and a magnetmounted on said frame adjacent to said coil to induce currents in it;and

first and second calibration coils mounted on opposite sides of saidmagnetically attractive material, for urging said boom to pivot in firstand second opposite directions, respectively, whereby to isolate themagnetic field produced by said first and second calibration coils fromsaid sensing coil.

8. The seismometer described in claim 7 wherein:

each of said calibration coils has a threadably mounted core ofmagnetically attractive material for screwing toward and away from saidmagnetically attractive material, whereby to enable adjustments to bemade so that each calibration coil produces the same output from saidsensing coil when the axis of pivoting of the boom is precisely verticalwith respect to local gravity.

A seismometer comprising:

a main frame;

boom assembly including a coil with first and second ends whereelectrical connections can be made;

a pair of hinges for pivotally mounting said boom on said frame, eachhinge including a pair of crossed resilient members of electricallyconductive material having opposite ends fixed to said boom and frame,respectively, said first and second ends of said coil electricallycoupled to different ones of said hinges; pair of output terminal meansmounted on said frame and electrically coupled to different ones of saidhinges; and

magnet means mounted on said frame adjacent to said coil for creating amagnetic field in the vicinity of said coil;

said boom including first and second mass members of electricallyconductive material electrically connected to different ones of saidpair of hinges, said mass members electrically insulated from eachother; and

said first and second ends of said coils being electrically coupled tosaid first and second mass members, respectively, so that each massmember electrically couples a coil end with a hinge.

10. The seismometer described in claim 9 wherein: each of said massmembers has a web region with a width and length each at least twice asgreat as its thickness, connecting a majority of the mass of the massmember to a hinge, each of said web regions oriented so that animaginary line normal to the faces of the web region is substantiallyparallel to the axis of pivoting of said boom, whereby to reduce thepossibility of damage to said hinge.

1. A seismometer comprising: a main frame; a boom; means for mountingsaid boom on said frame to allow pivoting of said boom with respect tosaid frame about a nominally vertical axis while resiliently urging saidboom toward a predetermined pivotal position; means for sensing theamount of pivoting of said boom; and first and second calibration meansfor urging said boom to pivot respectively in first and second oppositedirections, irrespective of the position of the boom whereby to enable ameasurement of the orientation of the axis of pivoting of said boom withrespect to the vertical.
 2. A seismometer comprising: a main frame;magnet means mounted on said frame; a flat substantially spiral coil; acoil frame of electrically insulative material extending about most ofthe periphery of said coil and supporting it; a pair of mass membersmounted at opposite faces of said coil frame, said mass membersconstructed of electrically conductive material, electrically insulatedfrom each other, and electrically coupled to different portions of saidcoil; a pair of hinges for pivotally coupling said mass members to saidmain frame so that said coil is positioned adjacent to said magnetmeans, said hinges constructed of electrically conductive material,electrically insulated from each other, and each electrically coupled tothe mass member which it piVotally couples to said main frame; and apair of output terminal means coupled to said hinges.
 3. The seismometerdescribed in claim 2 wherein: said coil frame has electricallyconductive layers on its opposite faces that are directly connected todifferent portions of said coil; and each of said mass members isconnected to a different one of said conductive layers, whereby tofacilitate construction of the seismometer.
 4. A seismometer comprising:a frame; magnet means mounted on said frame; a boom including a sensingcoil and a portion of magnetically attractive material spaced from saidcoil; means for pivotally mounting said boom on said frame so that saidsensing coil is urged toward a predetermined pivotal position where itlies adjacent to said magnet means while said portion of magneticallyattractive material is spaced from said magnet means; first and secondcalibration coils mounted on opposite sides of said portion ofmagnetically attractive material for respectively urging said boom topivot in opposite directions; and sensing terminal means electricallycoupled to said sensing coil.
 5. The seismometer described in claim 4wherein: said sensing coil and said portion of magnetically attractivematerial are on diametrically opposite sides of the axis of pivoting ofsaid boom.
 6. A seismometer comprising: a frame; magnet means mounted onsaid frame; and a boom including a coil adjacent to said magnet means,mass means for supporting said coil, and hinge means for pivotallysupporting said mass means on said frame, said mass means including apair of spaced web portions coupling the major portion of said massmeans to said hinge means, each web portion having a width and lengtheach at least twice as great as its thickness, said web portionsoriented so that imaginary lines normal to their faces are substantiallyparallel to the axis of pivoting of said mass means on said frame,whereby to protect said hinge means.
 7. A seismometer comprising: a mainframe; a boom; means for mounting said boom on said frame to allowpivoting of said boom with respect to said frame about a nominallyvertical axis while resiliently urging said boom toward a predeterminedpivotal position; said boom having first and second portions on oppositesides of its axis of pivoting, and said second portion includingmagnetically attractive material; means for sensing the amount ofpivoting of said boom, including a sensing coil mounted on said firstportion of said boom, and a magnet mounted on said frame adjacent tosaid coil to induce currents in it; and first and second calibrationcoils mounted on opposite sides of said magnetically attractivematerial, for urging said boom to pivot in first and second oppositedirections, respectively, whereby to isolate the magnetic field producedby said first and second calibration coils from said sensing coil. 8.The seismometer described in claim 7 wherein: each of said calibrationcoils has a threadably mounted core of magnetically attractive materialfor screwing toward and away from said magnetically attractive material,whereby to enable adjustments to be made so that each calibration coilproduces the same output from said sensing coil when the axis ofpivoting of the boom is precisely vertical with respect to localgravity.
 9. A seismometer comprising: a main frame; a boom assemblyincluding a coil with first and second ends where electrical connectionscan be made; a pair of hinges for pivotally mounting said boom on saidframe, each hinge including a pair of crossed resilient members ofelectrically conductive material having opposite ends fixed to said boomand frame, respectively, said first and second ends of said coilelectrically coupled to different ones of said hinges; a pair of outputterminal means mounted on said frame and electrically coupled todifferent ones of said hinges; and magneT means mounted on said frameadjacent to said coil for creating a magnetic field in the vicinity ofsaid coil; said boom including first and second mass members ofelectrically conductive material electrically connected to differentones of said pair of hinges, said mass members electrically insulatedfrom each other; and said first and second ends of said coils beingelectrically coupled to said first and second mass members,respectively, so that each mass member electrically couples a coil endwith a hinge.
 10. The seismometer described in claim 9 wherein: each ofsaid mass members has a web region with a width and length each at leasttwice as great as its thickness, connecting a majority of the mass ofthe mass member to a hinge, each of said web regions oriented so that animaginary line normal to the faces of the web region is substantiallyparallel to the axis of pivoting of said boom, whereby to reduce thepossibility of damage to said hinge.